In a high intensity discharge (HID) lamp, a medium to high pressure ionizable gas, such as mercury or sodium vapor, emits visible radiation upon excitation typically caused by passage of current through the gas. In the original class of HID lamps, discharge current was caused to flow between two electrodes. However, a major cause of early electroded HID lamp failure has been found attributable to at least two inherent operational characteristics of such lamps. First, during lamp operation, sputtering of electrode material onto the lamp envelope is common and reduces optical output. Second, thermal and electrical stresses often result in electrode failure.
Electrodeless HID lamps do not exhibit these life-shortening phenomena found in electroded HID lamps. One class of electrodeless HID lamps involves generating an arc discharge by establishing a solenoidal electric field in the gas; and, hence, these lamps are referred to as HID-SEF lamps. Unfortunately, HID-SEF lamps of the prior art have had limited applicability as described in U.S. Pat. No. 4,810,938, issued to P. D. Johnson, J. T. Dakin and J. M. Anderson on Mar. 7, 1989 and assigned to the instant assignee. As described in the cited patent, which is hereby incorporated by reference, one problem encountered in using electrodeless HID lamps is that their color rendering capability is inadequate for general purpose illumination. In particular, one requirement of general purpose illumination is that objects illuminated by a particular light source display substantially the same color as when illuminated by natural sunlight. A common standard used to measure this color rendering capability of a light source is the color rendering index (CRI) of the Commission Internationale de l'Eclairage (C.I.E.). For general lighting applications, a CRI value of 50 or greater is deemed necessary. Disadvantageously, color rendering capability of an HID lamp decreases with increasing efficacy. In the above-cited patent, however, it is recognized that a particular combination of fill materials can result in color improvement without adversely affecting lamp efficacy. Specifically, the lamp of the referenced patent utilizes a fill comprising sodium halide, cerium halide and xenon. Although at white color temperatures, this particular combination of fill ingredients provides improved efficacy and color rendition over the HID-SEF lamps of the prior art, it is desirable to find still other fill materials that will result in high efficacy and good color rendition.